Underwater connectors are utilized for structural terminations and circuit interconnections of electrical power and signal cabling systems. Such connectors can be found in underwater stationary platforms, submarines and other submersible vehicles or surface vessels, by way of example.
One common type of connector includes a metal shell (aka, connector plug, shell body or backshell) through which extends one or more conductors utilized for the transmission of electrical power or electronic or fiber-optic signals. These types of connectors are typically insulated from water and other environmental conditions by means of a polymeric encapsulating material, or boot, applied to the outside of the shell. Common polymeric, or rubber or rubber-like, encapsulating materials are polyurethane, polyethylene, or polychloroprene molding compounds. The encapsulating material typically seals the connector shell to its associated cable and is attached and made watertight to each by polymer-to-polymer or polymer-to-metal adhesive bonds. Typically, such connectors in present use are prone to premature failures due to degradation of the adhesive bond between the molded polymer boot and the metal shell. Bond degradation allows the development of leak paths for water to ingress into the connector resulting in loss of electrical resistance or overt short circuiting.
Certain environmental conditions can accelerate the rate of the types of degradation described above. Studies have shown that the most serious form of adhesive bond degradation occurs in seawater when metal connector shells are attached, or coupled, with dissimilar metals whereby a galvanic cell is formed. This commonly occurs when the couple includes the connector and a sacrificial anode, usually made of zinc or aluminum, which is attached to the hull of a ship, or other platform structure, to prevent corrosion. In such a galvanic cell the hull or platform structure and connectors attached to it act as cathodes. Although the cathodes are protected from corrosion, the galvanic cell sustains an electrochemical reaction that is deleterious to polymer-to-metal adhesive bonds. The process whereby the electrochemical reaction degrades adhesive bonding is commonly referred to as cathodic delamination.
Cathodic delamination is an underwater connector failure mode associated with the use of zinc anodes for corrosion control of marine metal structures. An electrochemical potential is developed between the hull zinc anodes and the (typically) stainless steel or Monel backshells of connectors which are electrically connected to the hull. Salt water provides the electrolyte function in the cell. This cathodic activity provides free electrons at the cathode (the back shell metal surface). The electrochemical reaction which takes place is essentially independent of the type of metal used in the back shell as long as the reaction potential is high enough that electrons are transferred from the zinc to the backshell. The effect of the free electrons at the surface of the backshell results in the reaction: EQU 1/2O.sub.2 +H.sub.2 O+2e.revreaction.-2OH
The hydroxide ions which result cause a local rise in pH which is concentrated at the surface of the backshell. This highly basic local concentration attacks the adhesive bond between the boot and backshell and can cause complete delamination at the metal to boot interface. In a connector, this loss of adhesive bond can lead to catastrophic flooding failure from leak paths created between the boot and the backshell.
Since hulls are polarized with sacrificial zinc anodes for corrosion protection, most dissimilar metals that are coupled to them (e.g., connector backshell plugs) form cathodes and are "protected" from corrosion in the same manner as the remainder of the hull. The electrochemical process is an oxidation/reduction reaction and there is no net accumulation of charge.
Electrochemistry
Connectors and similar hardware are subject to an electrochemical potential as part of a large galvanic cell. Sea water serves as the electrolyte. The hull is a first order conductor between the zinc anodes and the metallic backshell cathodes, seawater is the second order conductor. The sacrificial anode metal (zinc) strips off electrons and fine ions migrate into the seawater. Accordingly, free electrons gather at the backshell surface (cathode). FIG. 1A and FIG. 1B shows the process schematically. A variety of reactions at the interface between the more-noble metal backshell and electrolyte (seawater) are possible: EQU 2H.sub.3 O.sup.+ +2e.sup.- .revreaction.H.sub.2 +2H.sub.2 O Hydrogen evolution (1) EQU 1/2O.sub.2 +H.sub.2 O+2e.sup.- .revreaction.2OH.sup.- Hydroxide ion formation (2) EQU 2H.sup.+ +2e+1/2O.sub.2 .revreaction.H.sub.2 O Water formation (3) EQU M.sub.a O.sub.b +2bH.sup.+ +ae.sup.- .revreaction.aM+bH.sub.2 O Metal solvation (4) EQU M.sup.n+ +nOH.sup.- .revreaction.M(OH).sub.n Metal hydroxide (5)
Reaction (1) predominates whenever the electrochemical potential is high (-1 volt or more) vs std calomel electr-de and reaction (2) governs in low voltage (-0.8 volts) situations with dissolved oxygen present. At intermediate potentials, both reactions (1) and (2) may occur. Both reactions result in high pH conditions at the electrode (backshell). All of these electrochemical reactions occur on the surface; relative reaction rates vary according to physical conditions, species present at the interface (e.g., oxygenated seawater, water flow rate, and surface morphology).
These reactions are "catalyzed" by electron transfer, in other words, they occur only on surfaces where electrons are available. Reaction rates are limited by electron availability. Electrochemical reaction rate is termed exchange current density (i.sub.0). Exchange current densities vary widely for different metal surfaces. As an example, i.sub.0 for reaction (1) on copper, nickel, and iron are roughly three to four orders of magnitude higher than i.sub.0 for aluminum in similar electrolytes. Backshells are typically 316 stainless steel, Monel, or silicon aluminum bronze. The Monel connectors are chiefly copper and nickel and are accordingly more susceptible to cathodic delamination than many metals. In the case of backshells, selected materials are thought of as more noble and corrosion resistant. Nobel-metal material selection aggravates cathodic delamination since these materials are naturally proficient in electron exchange, thereby promoting galvanic reactions at the metal surface.
Reaction (2) causes a localized rise in hydroxide ions (rise in pH) at the connector/seawater interface that attacks rubber-to-metal adhesive bonds that in turn may create a leak path and subsequent electrical system failure.
In order to eliminate this cathodic delamination problem, glass reinforced epoxy (GRE) and other plastic or nonconductive connectors have been designed and utilized. However, these designs have limited applications since they typically require extensive development efforts for design, procurement and acceptance testing specifications. These types are also limited in some applications by their reduced mechanical strength and greater susceptibility to explosive shock, impact or other risks of random hazard damage typical in the marine environment.
U.S. Pat. No. 4,874,324 relates to connectors having a metal shell that includes a protective plastic coating of polyphenylene sulphide resin that is electro-deposited thereon, and over a portion of which a plastic coating or encapsulating boot is bonded. However, use of the type of organic material described in that patent as part of the metal coating has proved less than satisfactory in laboratory testing conducted for the Naval Research Laboratory's Underwater Sound Reference Detachment and the Naval Weapons Systems Center. During accelerated life testing with applied galvanic potential the material was observed to delaminate itself from the 316 L stainless steel connectors, to which it was applied prior to booting with polyurethane molding compound. Other organic coatings have likewise shown to either delaminate from the metal substrate, or connector backshell, or to suffer adhesive bond degradation with their encapsulating boots during accelerated life testing.
Bray et al. (September, (1993) MTS-93 Conference Proceedings), relates to steel substrates that have a glass and enamel cladding as two particular non-conductive coatings (NCC). Both of these materials were fused at high temperatures in an oven or furnace when applied to a particular substrate after being applied in a wet slurry, or slip. After firing, the materials were slow cooled to prevent breaking or delamination of the glass or enamel from the substrate because of a potential problem of thermal shock with these materials. The application techniques required that the metal substrates be raised to the same temperatures required to melt and fuse the glass or enamel materials. The process required to prepare surfaces with such NCC coatings has therefore in many cases proven to be an expensive, and commercially prohibitive process. In the sense that such treatments require high temperature equilibrium for both the NCC material and the entire device, for treatment of its outer surface, these processes also potentially damage the device, again making such a less desirable alternative.
U.S. Pat. No. 4,714,623 relates to surfaces onto which have been sprayed micronized ceramic, glass or carbon spheres into a wet or uncured resin prior to cure. After cure, the matrix is broken or altered to break into the voids formed by the spherical media giving a very toothy surface for the application of metal cladding. This type of bonding surfaces is reported to result in enhanced bonding.
U.S. Pat. No. 4,913,961 relates to a coating that is resistant to sulfur and ranadium compounds at elevated temperatures. (Scandia is present in the zirconia (a ceramic) between about 4 to about 8 mole percent.) Thermal spray coating is used particularly for the coating of superalloys useful for turbine blades and engine pistons.
U.S. Pat. No. 4,445,989 relates to metallic substrates having a layer of a conductive or ferrite or chromite. The ferrite or chromite layer serves as a sacrificial layer, and is described as having a thickness of at least 8 miles (203.2 mm) for this purpose. U.S. Pat. No. 4,578,114, relates to an aluminum and yttrium oxide thermally sprayed powder blend. The thermal spray of these blends onto metallic substrates is described as providing a coating having high temperature corrosion resistance and tenacity. These preparations include a metal alloy base together with aluminum and yttrium oxide, in amounts described by reference to weight percent of the metal alloy base, i.e., 1 to 15% aluminum and about 0.5% to 10% ytterium oxide by weight of the metal base alloy and other constituents of the composition.
These and other approaches fail to provide an economical and effective approach for inhibiting the breaches that occur between metal surfaces and metal to rubber surfaces. It is an object of the present invention to provide for coatings, methods of coating and coated devices that inhibit cathodic delamination from metal surfaces, such as those common to metal connectors, and to also enhance bond strength between metal and rubber surfaces. It is a further object of the present invention to provide metal devices having improved connector service life, particularly in the field of connectors that are routinely used in a marine environment. It is a further object of the present invention to provide a more economical method of providing a nonconductive coating to a metal surface, without loss in bond strength or significant change in thickness of the surface being treated. It is also an object of the present invention to provide a solution to the problem associated with useful life of currently used underwater electrical connectors having polyurethane, polychloroprene and other polymeric molded boots or coverings.
It is further an object of the present invention to provide improved non-conductive metal coatings that prevent the electrochemical current flow at metal surfaces and essentially halt hydroxide ion production, and thus the damage such electro chemical current flow induces.
Solutions provided by the various aspects by the present invention address long recognized problems currently experienced in many Navy and industrial marine applications, cathodic delamination being the single most important problem currently described in the technology of underwater connectors. A solution to this historic problem at low cost would also serve other long-felt needs in these and many industries where metal surfaces are involved, such as on the tube seals in the tension leg assemblies used for offshore drilling platforms.